[exim] HArdware Suggestions for exim courier-imap

Hello all,

I have been using Qmail and courier-imap for about 3 years now and it
has come time to install a bigger server.
I have been finding that Exim has many features that Qmail lacks so it
is also time to move to Exim.

We currently have a Dual P3 700 with 6 hard disks with 600 accounts on
it. We have moved webmail of to another server but Imap and Smtp is
still pushing the 15min load average past 6.00 .
We have about 100 users with Outlook using Imap and the rest use Horde
webmail.

With so many users is there anyone who can give me a few pointers as to
how big to go on the new server? I understand that disk IO is the big
thing with Imap so we are looking at Raid but not sure whether to move
from raid5 to another faster option. Also CPU's and Ram. We are looking
at Dual 3Gig with 4Gig of Ram.

Any pointers or help would be greatly accepted.

Simon